% !TeX program = xelatex
% !TeX encoding = UTF-8
\documentclass[UTF8]{standalone}
\usepackage{amsmath,newtxmath,esint,ctex,tikz}
\begin{document}
\begin{tikzpicture}
	\draw[domain=0:3.2,smooth,samples=100]  plot[id=1] function{(x*(3-x)**2/9)**0.5};
	\draw[domain=0:3.2,smooth,samples=100]  plot[id=2] function{-(x*(3-x)**2/9)**0.5};
	\draw[-latex] (-1,0) -- ++ (5,0) node[below] {$x$};
	\draw[-latex] (0,-1) -- ++ (0,3) node[right] {$y$};
	\node[below=5pt,left=-3pt] at (0,0) {$O$};
	\node[below] at (3,0) {$3$};
\end{tikzpicture}
\end{document}